OSA/ CLEO 2011
CWO5.pdf
Spatio-temporally Focused Femtosecond Laser Pulses for
Anisotropic Writing in Optically Transparent Materials
Dawn N. Vitek,1 Erica Block,1 Yves Bellouard,2 Daniel E. Adams,3 Sterling Backus,4 David Kleinfeld,5
Charles Durfee,1 and Jeffrey A. Squier1,*
1
Center for Micro-integrated Optics for Advanced Bio-imaging and Control, Department of Physics, Colorado School of Mines, Golden,
Colorado 80401, USA
2
Department of Mechanical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
3
Department of Physics and JILA, University of Colorado at Boulder and National Institute of Standards and Technology, Boulder, Colorado
80309, USA
4
Kapteyn-Murnane Laboratories, Inc., Boulder, CO 80301, USA
5
Department of Physics, University of California at San Diego, La Jolla, California 92093, USA
*
jsquier@mines.edu
Abstract: Simultaneous spatial and temporal focusing provides precise control of the pulse front
tilt necessary for anisotropic writing and maintains this behavior over a large range of focal
positions and at low numerical aperture and fluence.
©2011 Optical Society of America
OCIS codes: (050.6875) Three-dimensional fabrication; (230.4000) Microstructure fabrication.
Femtosecond laser material processing has yielded structures with remarkable properties that, when combined, can
produce devices with spectroscopic, fluidic and mechanical microscale functionalities. Depending on the substrate
and the laser parameters, the modification may appear as a change in the refractive index, a void or a birefringent
feature [1]. Birefringent features have been, thus far, less frequently applied in microscale functionalities, but they
were recently demonstrated for use in unique applications including a polarization-selective optical router [2] and a
polarization diffraction grating [3]. Under some conditions these birefringent features have the additional quality of
dependence upon the sample’s scanning direction in that the characteristics of the modification change when the
scan direction is reversed. This phenomenon is due to pulse front tilt (PFT) [1,4]. In practice, PFT may be imposed
on the laser beam by tuning the laser’s grating compressor [1]. However, any adjustment to the parallelism of the
gratings produces angular dispersion that necessarily results in spatial and temporal distortions at focus.
Consequently, the conditions for generating PFT are not easily translated between systems so that deciphering the
effect of PFT on anisotropic writing is confounded.
We present a method to obtain the conditions necessary for anisotropic writing in a controlled and reproducible
manner by exploiting the PFT inherent to a simultaneous spatial and temporal focusing (SSTF) arrangement [5]. In
SSTF lateral spatial chirping is used to form a frequency-distributed array of low numerical aperture (NA) beamlets.
A feature of SSTF is that out-of-focus nonlinear interactions are suppressed, facilitating femtosecond material
modification in the bulk of transparent substrates [6]. Importantly, at focus, the frequencies overlap spatially and
temporally, and the pulse is transform-limited in time as well as diffraction-limited in space. Thus, the PFT
generated in an SSTF system does not suffer from the distortions associated with misaligning the compressor. The
striking contrast generated by PFT is shown in Fig. 1. Anisotropic writing is observed for writing directions parallel
to the axis of PFT (Fig. 1(c)-(e)) but not for directions perpendicular to the axis of PFT (Fig. 1(b)).
Fig. 1. (a) The orientation of the pulse front tilt at the focal plane (dashed line). The pulse arrives first on the left hand side of the focal spot. In
(b)-(e) blue arrows indicate the lateral scanning direction for the laser beam with respect to the sample. In (b) the scanning directions were
OSA/ CLEO 2011
CWO5.pdf
perpendicular to the PFT. In (c)-(e) the scanning directions were parallel to the axis of PFT. Each set of anti-parallel lines was imaged with bright
field (top) and cross-polarized illumination (bottom). The orientation of the electric field, E, is marked with black arrows. In (b)-(d) the scanning
rate was 10 µm/s, and the location of the axial focal position was in the bulk of the sample, 284 µm beneath the front surface. In (e) the scanning
rate was 5 µm/s and the focal position was 40 µm beyond the back surface of the sample. Scale bar, 50 µm. (f) A scanning electron microscope
image of a Chevron-shaped structure. Scale bar, 10 µm.
The magnitude of PFT in an SSTF system is readily adjusted by changing the beam aspect ratio (BAR), i.e., the
ratio of the beam width in the spatially chirped dimension to the beam width in the unchirped dimension. Fourier
beam propagation simulations predicted that the PFT changed proportionally with the BAR (Fig. 2(b)). We
examined anisotropic writing for a PFT value of 16,000 fs/mm (Fig. 2(a)), which was five orders of magnitude more
PFT than employed in a study by Yang et al. [1]. Notably, our experiments showed a reduced dependence on the
axial focal position in the sample compared to Yang et al., who reported that 5 µm movement away from the
optimum focal position eliminated the appearance of anisotropic writing [1]. Under our experimental conditions
(0.88 J/cm2, 0.03 NA), we observed anisotropic writing over a range of axial focal positions from 230 µm to 500 µm
beneath the surface of a 500 µm thick fused quartz sample. Additionally, anisotropic void formation was revealed on
the back surface of the sample (Fig. 1(e), (f)) where either circular wells or Chevron shapes (the shape of a ‘V’
character) were ablated, depending on the scanning direction.
Fig. 2. (a) The simulated spatio-temporal intensity profile of the pulse at the focal plane where
x is the spatial dimension along the direction of spatial chirp. (b) The predicted relationship
between the pulse front tilt (PFT) and the beam aspect ratio (BAR) for our system.
In summary, with an SSTF system, the PFT necessary in anisotropic writing is achieved in a calculable and
reproducible manner and without the need to misalign the laser’s grating or prism compressor. The SSTF system,
having several orders of magnitude larger PFT and much lower NA, maintained anisotropic writing over a greater
range of focal depths (230 µm to 500 µm), including the back surface of a 500 µm thick sample, and at more than
two orders of magnitude less fluence than reported by previous work. We anticipate that the anisotropic surface
patterning possible with SSTF will broaden applications for femtosecond laser fabrication. Notably, the structures
we have created here are of sufficient extent that Chevron-patterned channels can be created in a single pass for
particle alignment in microfluidic devices [7]. Thus, devices can be fabricated quickly and efficiently.
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Acknowledgements
We thank Mariana Potcoava for her assistance with imaging. We gratefully acknowledge support for this work
from the AFOSR grants FA9550-07-10026 and FA9550-10-C-0017 and the NIH EB003832.